This invention relates to an improved solid state sensing device for selectively detecting the presence of gases within an atmosphere, particularly refrigerant gases such as hydrofluorocarbons (HFC's) and sulphur hexafluoride.
There are many instances where it is necessary to detect the presence of specific gases or vapors in an atmosphere. The gas or vapor may be harmful to human health and thus it may be desirable to monitor a work environment to insure that the concentration of the gas or vapor does not exceed some threshold level. Or, it may be desirable to test pressurized equipment for leaks which could indicate a malfunction. Particular types of gases or vapors which it is desirable to be able to detect are halogen containing gases such as those used as refrigerants and fire extinguishants. For proper operation, leaks from refrigeration, air conditioning, and fire extinguishing equipment must be eliminated or kept to a minimum. Thus, it is necessary to test both components and final assemblies for leaks.
The widely used chlorofluorocarbon (CFC) refrigerants R-12 and R-11 and the fire extinguishants HALON 1301 and HALON 1211 are among the group of chlorinated and/or brominated compounds being phased out or severely restricted by international agreement due to their potential for destroying ozone in the upper atmosphere. The substitutes for these materials in most applications are either hydrofluorocarbons (HFC's) which have no ozone depletion potential or hydrochlorofluorocarbons (HCFC's) which have a significantly reduced ozone depletion potential. For example, the HFC, R-134a, is replacing R-12 in refrigeration and air conditioning applications where stringent leak testing is required to insure adequate performance of the equipment. Suitable means for detecting leaks is a critical factor in the conversion of this industry to non-ozone depleting chemicals. Although present state of the art gas sensors are quite effective for detecting chlorinated and brominated gases (CFC's, HCFC's, and HALONS), they are not adequate for detecting HFC's.
The prior art of sensors for detecting gases or vapors in an atmosphere has utilized a number of different detection principles. These include 1) measuring changes in the rate of heat loss from a circuit component resulting from changes in thermal conductivity of the atmosphere being sampled 2) electron capture methods which measure ions formed by the gas of interest when it is exposed to low energy electrons 3) positive ion emission methods in which the gas of interest facilitates emission of positive ions from a metal surface in the presence of a strong electric field, and 4) depletion layer methods in which the gas of interest alters the concentration of charge carrying species in a surface depletion layer and thus changes the internal resistance of the sensor.
These prior art sensors all have problems in regard to detection of HFC's. Thermal conductivity detectors are not specific to a given gas and thus are likely to give false responses. They also do not have as high a sensitivity as other types of detectors. Electron capture detectors use a radioactive source which requires special handling and in most cases must be licensed by governmental authorities. Prior art positive ion emission and surface depletion layer detectors have very low sensitivity to HFC's. One way to increase their sensitivity to HFC's is to increase the sensor operating temperature. However, this has the adverse effects of significantly shortening the life of the sensor and greatly increasing its sensitivity to other gases, particularly to chlorinated and/or brominated gases. This extreme sensitivity to minor impurities in the background atmosphere makes existing sensors unsuitable for detecting HFC's.
A solid state sensor having the ability of detecting the presence of many undesirable gases and vapors within an atmosphere is disclosed by Loh in U.S. Pat. No. 3,751,968. A solid state element, which contains alkali metal ions which readily accept negative ions of the subject gases and vapors, is brought into reactive contact therewith. The element is specially prepared to create an outer layer along its boundaries that is depleted of ions. The conductivity of the heated element in an atmosphere free of the reactive gases and vapors is low. However, the presence of one or more of the reactive gases and vapors causes ions to flow across the depletion boundary and increases the conductivity of the element. Electrical circuit means are provided for detecting an increase in the conductivity of the element and generating a signal indicative of the presence of a reactive constituent in the test atmosphere.
The Loh type device has proven to be an extremely useful tool for sensing the presence or absence of a halogen gas within a specific atmosphere. Special applications include leak detection in refrigeration equipment and the presence of potentially dangerous gases within an operating room or the like.
However, many test atmospheres contain more than one constituent that can react with the sensing element and, as a result, unwanted interference signals are sometimes generated that make it difficult to discern the presence of a single gas or vapor of immediate interest. Water vapor, which is ordinarily present in air, has the ability to trigger the sensor and has proven to be troublesome when air sampling is required. Hydrocarbons and chlorine atom containing cleaning solvents such as trichloroethylene have proven to be troublesome background gases commonly found in industrial environments. Because most prior art sensors have low sensitivity to gases which do not contain chlorine or bromine atoms, such as HFC's or sulfur hexafluoride, it has been almost impossible to detect the presence of these gases, particularly when interfering gases are present. The device is conventionally operated within the range of between 700.degree. C. and 850.degree. C. Increasing the sensor operating temperature increases the signal due to gases such as HFC's or sulfur hexafluoride, but also amplifies the unwanted interference signals thus negating any potential benefit. Increased operating temperature also significantly reduces the useful lifetime of the sensor.